The present invention relates to a rare earth permanent magnet for use in rotary equipments, electronic parts or components, electronic equipments, etc.
A ferrite magnet has been widely used in a permanent magnet rotary equipment due to its low cost. However, a rare earth magnet has recently come to be used in place of the ferrite magnet to meet the recently increasing demand for reducing the size and improving the efficiency of the rotary equipment. For example, an Sm--Co magnet having a good heat stability and corrosion resistance and a high-performance Nd--Fe--B magnet are used depending on the application use. Of the Sm--Co magnets, Sm.sub.2 Co.sub.17 magnet is advantageous because the properties thereof is not deteriorated by machining and the protective coating is practically not necessitated. Formerly, the maximum energy product, (BH)max, of the Nd--Fe--B magnet was about 35 MGOe at most. However, at present the (BH)max of the Nd--Fe--B magnet has reached 40-45 MGOe. In addition, although the former Nd--Fe--B magnet involved a problem of insufficient heat resistance, this heat resistance has been improve to a certain extent in some application use.
However, the Nd--Fe--B magnet still involves various disadvantages to be removed in view of practical use, such as a large thermal coefficient of the residual magnetic flux density (Br) and the coercive force (iHc), a low Curie temperature, unavoidable protective coating of the magnet surface and a low electrical resistance.
Of the above disadvantages, the low electrical resistance is the most difficult one to be solved because the rare earth magnet comprises electrical conductive metallic substances. If the electrical resistance is low, the rare earth magnet generates a large amount of heat due to eddy current to reduce the efficiency of a rotary equipment such as a motor, when used under a condition in which the amount of magnetic flux changes periodically. Although it is difficult to increase the electrical resistance of a metallic substance, it has been expected that the application field of the rare earth magnet can be more expanded if the electrical resistance thereof is increased. To increase the electrical resistance of a magnet made of a metallic material, JP-A-4-125907 teaches to deposit by sputtering an insulating thin film such as SiO.sub.2 film on fine powder of a metal such as Fe--Co alloy, and sinter the resultant powder. JP-A-5-121220 teaches to coat a resin-bonded magnet powder with an inorganic binder by sol-gel method and subject the resultant powder to a direct compacting in a molding die while passing electrical current, thereby producing a full density magnet. However, the characteristics of the rare earth magnets reported therein are still insufficient and necessary to be further improved.